The invention relates to mass spectrometers and also to methods of ion separation and ion detection for use with mass spectrometers.
A mass spectrometer is capable of ionising a neutral analyte molecule to form a charged parent ion that may then fragment to produce a range of smaller ions. The resulting ions are collected sequentially at progressively higher mass/charge (m/z) ratios to yield a so-called mass spectrum that can be used to “fingerprint” the original molecule as well as providing much other information. In general, mass spectrometers offer high sensitivity, low detection limits and a wide diversity of applications.
There are a number of conventional configurations of mass spectrometers including magnetic sector type, quadrupole type and time of flight type. More recently, one of the present inventors has developed a new type of mass spectrometer that operates according to a different basic principle, as described in U.S. Pat. No. 7,247,847 [1], the full contents of which are incorporated herein by reference. The mass spectrometer of U.S. Pat. No. 7,247,847 accelerates all ion species to nominally equal velocities irrespective of their mass-to-charge ratios to provide a so-called constant velocity or iso-tach mass spectrometer. This is in contrast to time-of-flight mass spectrometers which aim to impart the same kinetic energy to all ion species irrespective of mass.
U.S. Pat. No. 7,247,847 discloses two principal embodiments which differ in respect of their detector designs. These two prior art designs are reproduced in FIGS. 1 and 2 of the accompanying drawings.
In both FIG. 1 and FIG. 2, a mass spectrometer 10 is shown comprising three main components connected serially, namely an ion source 12, a mass filter 14 (sometimes referred to as an analyser) and an ion detector 16.
In the FIG. 1 design, the ion detector 16 comprises a detector array 56 and an ion disperser to disperse the ions over the detector array according to their mass-to-charge ratios. The ion disperser comprises electrodes 52, 54 that produce a curved electric field which deflects the ions onto the array by amounts depending on their energies, which in turn depend on their mass-to-charge ratios. The least energetic (lowest mass) ions are deflected through the largest angle and the most energetic ions (highest mass) through the smallest angle. Consequently ions are dispersed spatially from left to right as viewing FIG. 1. It is noted that this type of dispersion ideally requires the ions to have an infinitely thin rectangular cross-section prior to deflection. In reality, the ion beam generated by the ion source 12 and mass filter 14 has a circular cross-section and this limits resolution of the detector. The resolution can be improved by clipping the ion beam with an ion absorbing slit placed in the beam path, but this means that some of the ions are lost to the detector, thereby reducing sensitivity. A trade-off between resolution and sensitivity thus pertains.
In the FIG. 2 design, an alternative ion detector 16 is used which comprises a first detector electrode 60 which is annular with an aperture for the passage of ions. This electrode 60 acts as an energy selector. Following this, a second detector electrode 62 is located in the ion path. This is a single element detector, such as a Faraday cup. A voltage supply 63 is provided for applying voltages to the first detector electrode 60 and the second detector electrode 62. In use, the first detector electrode 60 and the second detector electrode 62 are set to a potential of Vt+Vr volts, where Vt is the time varying voltage profile as defined above, and Vr is a bias voltage selected to repel, or reflect, ions having energies less than Vr electron volts. Hence, only ions having energies equal to or greater than Vr electron volts pass through the first detector electrode 60 and reach the second detector electrode 62 for detection.
To obtain a set of mass spectrum data, Vr is initially set to zero, so that all the ions in a packet are detected. For the next packet, Vr is increased slightly to reflect the lowest energy ions, and allow the remainder to be detected. This process is repeated, with Vr increased incrementally for each packet, until the field is such that all ions are reflected and no ions are detected. The data set of detected signals for each packet can then be manipulated to yield a plot of ion current against m/z ratios, i.e. the mass spectrum. This configuration allows for a simple and compact linear construction. However, the voltage sweeping process means that a large proportion of the ions is rejected, so sensitivity is reduced. The design also suffers from noise in that there is an uninterrupted direct path along the beam axis from the ion source 12 and mass filter 14 into the detector 16. Consequently, energetic photons produced inside the ion source are incident on the detector and can cause false counts. Moreover, non-ionised atoms and molecules, so-called neutrals, that are generated by energetic ions that pass sufficiently close to the grid to be discharged, but not significantly deflected off-axis, may also impinge on the detector and cause false counts.
It would therefore be desirable to improve the detector design of mass spectrometers operating according to the constant velocity or iso-tach principle.